The present invention relates to a pressure developing device, including a pair of pressure developing rollers, employed in a printer or copying machine which uses a photo and pressure sensitive recording sheet, more particularly to a pressure developing device further including backup rollers for pressurizing each of pair of pressure developing rollers, while being arranged in such a manner that a pressure force generated between the pair of pressure developing rollers is adjusted from the outside of the pressure developing device.
Conventionally, this type of pressure developing device is provided with only a pair of pressure rollers rather than with a pair of backup rollers arranged to be brought into and out of contact with each of the pair of pressure developing rollers. Therefore, to withstand a high load applied in a pressure developing process, large diameter pressure rollers for pressurizing the pressure and photo sensitive recording sheet are used. In addition, the pair of pressure rollers are brought into and out of contact with each other and rotatably supported in a predetermined position of a housing of the pressure developing device. Thereby, the pressure rollers are not arranged to be brought into and out of contact with each other.
However, when conducting a pressure developing operating using a micro-capsule sheet disclosed in Japanese Patent Provisional Publication SHO No. 58-88739 which had been filed by the Mead Corporation(the corresponding U.S. application is U.S. Pat. No. 4,399,209), for example, as the photo and pressure sensitive recording sheet, a load of 700 kg/cm.sup.2 or more should be applied. It is around twice the pressure force necessary for pressuring and fixing the conventional toner images. Therefore, in the conventional pressure developing device described above, unless diameters of the pair of pressure rollers are very large, the pair of pressure rollers are deformed in the pressure developing state, i.e., a pressure force is generated between the pair of pressure developing rollers. Therefore, an equal pressure force along a longitudinal direction of the pair of pressure developing rollers cannot be applied to the micro-capsule sheet. In addition, when the diameter of the pressure rollers are very large, the pressure developing device also becomes large and the weight also increases.
Further, in the conventional pressure developing device, it is difficult to set a pressure force accurately with a simple operation. For generating a pressure force, an elastic spring arranged to be brought into contact with the pressure developing rollers is generally employed. This type of the pressure developing device is disclosed in Japanese Patent Provisional Publication SHO No. 62-30626. In this type of pressure developing device, a spring supporter is composed of a bolt which is concentrically fastened to a frame and a washer disposed between the bolt and the spring. The bolt is contacted with the other end of the spring through the washer at one end so as to keep applying a predetermined amount of load, i.e., a predetermined pressure force, to the spring. At the center of the bolt, a through hole which is axially open is formed. When adjusting the pressure force, the entire pressure developing device is set to a jig main unit. After that, a piston rod of a hydraulic cylinder is inserted into the through hole of the bolt and contacted with the washer, thereby compressing the spring through the washer. On the hydraulic cylinder, a hydraulic pressure gauge is provided. In accordance with the amount of the hydraulic pressure, the spring load can be determined. When a predetermined amount of load is produced by the spring, an operation of the hydraulic cylinder is stopped and the bolt is screwed until it is contacted with the washer, thereby keeping the spring load to the predetermined value.
However, it is difficult to set the pressure developing device to the jig main unit. In addition, a stroke of the piston rod is large, so that a size of the hydraulic cylinder becomes large. The piston rod is inserted from the through hole of the bolt to the frame and a load is applied to the spring through the washer. However, when the frame is set to the jig main unit, it is necessary to escape the frame, bolt, and so forth in proper positions so that they do not interfere with the piston rod. Thus, it is difficult to set the pressure developing device to the jig main unit. In addition, since the distance between the escape position of the piston rod and the operation position is large and thereby the stroke of the piston rod increases, the hydraulic cylinder, that is, the size of the entire adjustment jig becomes large, resulting in increasing the weight, cost, installation space, and so forth. Moreover, it is also necessary to improve the positioning accuracy of the jig main unit of the pressure development apparatus for inserting the piston rod into the through hole of the bolt. Thereby, the adjustment work becomes complicated.